Case study 1
This was a system for a typical, small,
pre-fabricated fast food restaurant.
It is assumed the system only serves
the hot water demand.
The life cycle analysis includes the
initial capital costs, projected annual
fuel costs, and estimated annual
servicing costs which are assumed to
increase by 2% each year. The annual
fuel cost assumes the same daily fuel
consumption over the whole year.
The servicing cost is based on £130/
year per boiler, and £160/year for a
boiler and cylinder. The resulting net
present values of the three showed
that the continuous flow system is
6 - 7% lower than the two storage
systems based on that 20-year
analysis period.
Case study 2
This was a system for a shower block
in a holiday camp, with six showers
and four basin taps.
For the continuous flow option, the
analysis is based on a configuration
of 4 continuous flow water heaters
that each have a nominal output
of 48 kW, which, for this particular
application, because there is a very
short run of pipe work between the
water heaters and the fixtures, it is
possible for the water heaters to
generate water at 40°C. The risk of
legionella is overcome by regular
flushing of the system.
For the energy required to deal with
the instantaneous hot water demand
the analysis assumes that the bulk
of the hot water demand would be
due to the showers. Each shower
would typically have a flow rate of
around 9 litres per minute, which if
mixed to 40ºC would equate to an
instantaneous load of around 19 kW
each, so even if there was only one
shower running, the load on a single
continuous flow water heater would
equate to an efficiency of 95% based
on the performance curve. As the hot
water demand increases, the modular
nature of this particular configuration
would allow the 95% efficiency to
be maintained for the bulk of the
demand.
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The difference in daily fuel costs
between the three systems is around
7%. The usage profile for this
case study is particularly suited to
continuous flow systems (i.e. high
but infrequent demand over the day),
which is why there is an advantage
in fuel consumption over the storage
systems.
SYSTEMS EMPLOYED IN THE STUDY
* Indirect fired storage systems
This consists of separate heat source
and storage cylinders, where for this
study the heat source will be gas
fired boilers running on 80ºC flow
and 60ºC return with gross efficiency
of 89%. As the hot water is stored at
60ºC in insulated cylinders there will
be standing heat losses. The standby
electricity is assumed to be 15W per
boiler, and 60W when operating to
cover fan and controls.
Direct fired storage systems
This is where the hot water storage
cylinder has an integral gas burner to
directly heat the water, and the key
difference with indirect fired storage
systems is that they are designed
to operate in condensing mode and
generally achieve gross efficiency
up to 96%. A disadvantage of direct
fired storage systems is that the
standing losses are around 3 times
higher than indirect storage cylinders.
Continuous flow systems
This is essentially a gas boiler that
is designed to instantaneously heat
mains water for supply directly to
water fixtures without any storage.
The key advantages with this type
of hot water heating, is firstly saving
in space needed for hot water
cylinders and the associated standing
heat losses. The challenge is that
instantaneous hot water demand
can vary hugely for most systems
and therefore the continuous flow
heating system needs to have a
wide modulation range as well as
maintaining efficiency over that
range.
For more information on the RINNAI
product range of high efficiency
condensing continuous flow hot
water heating units and systems
visit www.rinnaiuk.com